Hello all, below I've provided some info, so as to frame my question:

I've had my stock LS650 Rear Hub laced to a 1979 GS550 steel 18" X 2.15" rim. I've got a Coker 450X18 tall wall tire on it and the total height of the rear wheel is now 24". The stock rear wheel combo (with a 140/80-15 Metzler tire) is 21" tall in height. I've converted my S-40 to the chain drive and am running a 17 Tooth front and 47 tooth rear sprocket for a rotational ratio of 2.76 : 1...(with stock tire) This ratio works well and allows me to use 5th gear as an overdrive gear and the bike now can cruise at about 80mph all day long, with a wee bit of passing power still left...

My question is this: If I've increased the height of the rear wheel from the 21" overall stock height (as measured with a Metzler 140/80-15 tire on the stock 15" wheel) to the new height of 24" (as measured with the GS550 18" and Coker tall wall tire), don't I have to decrease the size of the rotational ratio, in order to keep the same effect that I get with the stock wheel/tire combo that I'm running with the 17T & 47T sprockets? I figure I do have to decrease the sprocket size(s), due to the 18" GS550 wheel/tire combo being taller than the stock wheel/tire combo...but to what sprocket size(s) do I need to decrease down to? Do I simply just calculate the percentage that the wheel height has increased from stock, than just decrease the 2.76 : 1 ratio that I'm running there, by the same percentage? I know that I have to run smaller sprockets but to avoid having to buy a ton of wrong sized ones, I'm trying to see if there's just a formula to go by in doing a chain conversion while increasing the rear wheel height???

By the way, the 18" wheel is for a Voodoo Vintage based hardtail frame that I'm currently slapping together in hopes to enter it in the Stampede  cross country rigid race next year...If anyone is interested, I can post pictures and commentary along the way during the build. For now, I'm hoping somebody understands what it is I'm asking with the above spewage??? Thanxaton-thumps

Check your measurements
Dunlop states the 140/90-15 as being 24.73" in diameter.
And I was seriously doubting that your 18" wheel was going to fit until you stated you've got a hardtail.
I've got a 25.45" tire in it and seriously doubt you can go much taller.

...also,.. plug your info in here...
http://gearingcommander.com/

Same here! Pics please.